WASHINGTON, Fed. 21 (CNBC) Federal prosecutors in New York on Wednesday said they charged a Japanese Yakuza leader with conspiring to traffic nuclear materials from Burma to other countries in the belief that they would be used by Iran to make a nuclear weapon.

The accused gangster, Takeshi Ebisawa, "and his confederates showed samples of nuclear materials in Thailand" to an undercover agent from the U.S. Drug Enforcement Administration who was posing as a narcotics and weapons trafficker with access to an Iranian general, prosecutors said.

“With the assistance of Thai authorities, the nuclear samples were seized and subsequently transferred to the custody of U.S. law enforcement,” the U.S. Attorney’s Office in Manhattan said in a statement announcing a superseding indictment against Ebisawa and another man.

“A U.S. nuclear forensic laboratory later analyzed the samples and confirmed that the samples contain uranium and weapons-grade plutonium,” the statement said.

The indictment says that in September 2020, Ebisawa emailed the undercover DEA agent a letter in the name of a mining company offering to sell 50 metric tons of uranium and thorium for $6.85 million.

U.S. Attorney Damian Williams said, “It is impossible to overstate the seriousness of the conduct alleged in today’s indictment.”

Williams said Ebisawa “brazenly trafficked” the nuclear material while believing it would be used to develop a nuclear weapons program.”

The top prosecutor also said that even as he tried to sell the nuclear materials, the Yakuza leader “also negotiated for the purchase of deadly weapons, including surface-to-air missiles,” M60 machine guns, AK-47s and armor-piercing ammunition.

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